Optical communications system

ABSTRACT

When a branching unit combines a first optical signal transmitted from a branch station with a second optical signal which is different in power level from the first optical signal and is transmitted from a terminal station A or B in an optical add-drop system, the S/N ratio of the lower power level of the two different power levels decreases, thereby deteriorating the system performance. Therefore, a dummy light is transmitted together with an optical signal to adjust the power level of the optical signal. Otherwise, an optical attenuator or an active optical signal level adjustment unit is provided for the branching unit so that both optical signals to be combined can be equal in level.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical communications systemapplicable to long-distance communications such as underseas cablecommunications, etc.

[0003] 2. Description of the Related Art

[0004] Recently, optical communications systems have been widelydeveloped to realize large-capacity and high-speed communicationssystems. Especially when a large volume of information is to besimultaneously transmitted, an optical wave-length multiplexing systemis highly evaluated and is studied for practical use in the near future.In the optical wave-length multiplexing system, an optical signal whichcarries information and have a plurality of wavelengths iswavelength-multiplexed for transmission. An optical signal of each wavelength corresponds to at least one communications channel. In theoptical wavelength multiplexing system applicable to the long-distancecommunications such as underseas cable communications, an opticaladd-drop system is under development in which an optical signal having aspecific wavelength or an optical signal along a specific channel amongoptical signals wavelength-multiplexed in the communications line isbranched to transmit an optical signal along a channel branched to aterminal station, and the optical signal transmitted from the terminalstation with the same wavelength as the branched channel is combinedagain to the optical signal transmitted through the originaltransmission line for transmission to the terminating station.

[0005]FIGS. 1A through 1F show the conventional optical add-drop systemand the problem with the system.

[0006]FIG. 1A is a block diagram showing the entire configuration of theoptical add-drop system. The basic configuration in the optical add-dropsystem has a terminal station A as a transmitting station fortransmitting an optical-wavelength multiplexed optical signal, aterminal station C as a receiving station for receiving a signal fromthe terminal station A, a branching unit 1100 for branching or combiningan optical signal of a specific wavelength in the optical signals fromthe terminal station A, and a terminal station B for receiving theoptical signal branched by the branching unit 1100, and transmitting newinformation with an optical signal having the same wavelength as thereceived optical signal. Normally in the underseas cable communications,the branching unit 1100 is mounted underseas to transmit optical signalsto, for example, the terminal stations A, B, and C provided in differentnations. Typically, the distance between the terminal stations A and Cis approximately 3,000 km, and the branching unit 1100 is providedaround the central point between these stations. Since the intensity ofan optical signal is attenuated when the optical signal is transmittedfor a long distance, the transmission lines between the terminal stationA and the branching unit 1100, between the terminal station B and thebranching unit 1100, and between the terminal station C and thebranching unit 1100 have a plurality of optical amplifiers 1101, 1102,and 1103 respectively. FIG. 1A shows the optical amplifiers 1101, 1102,and 1103 apiece for respective transmission lines for a simpleillustration, but there are actually much more optical amplifiers foreach transmission line. Normally, each of the optical amplifiers 1101,1102, and 1103 has an automatic output level control circuit (ALCcircuit) to keep the output level of each of the optical amplifiers1101, 1102, and 1103 constant so that the optical signal can beconstantly amplified to a specific output level.

[0007]FIG. 1A shows the transmission line for one-way communications.Actually, the circuit is designed to establish two-way communications,that is, up-line and down-line communications.

[0008]FIGS. 1B through 1F show an optical signal and its problem in eachtransmission line.

[0009]FIG. 1B shows the optical signal at point A in FIG. 1A. In thecase shown in FIG. 1B, optical signals having four different wavelengthsare wavelength-multiplexed and transmitted from the terminal station A.The mound under each optical signal is called an amplified spontaneousemission (ASE) noise. It is produced when a noise superposed to anoptical signal is amplified with the optical signal by an opticalamplifier. The characteristics of the operations of the opticalcommunications system depend on the S/N ratio of the optical signal tothe ASE.

[0010] In the branching unit 1100, the optical signal having awavelength λ₁ is branched and transmitted to the terminal station B, andan optical signal having the wavelength λ₁ is transmitted from theterminal station B to the terminal station C.

[0011] An optical signal having a wavelength other than wavelength λ₁ inthe signal (FIG. 1B) transmitted from the terminal station A is notbranched by the branching unit 1100, but is transmitted as is to theterminal station C. The terminal station B receives the optical signalhaving wavelength λ₁ and transmits an optical signal having the samewavelength λ₁. FIG. 1C shows the state at point B of the signaltransmitted from the terminal station B and amplified by the opticalamplifier 1102. The branching unit 1100 combines the optical signalhaving wavelength λ₁ transmitted from the terminal station B with thelight having wavelength λ₂ through λ₄, and transmits the result to theterminal station C.

[0012]FIG. 1D shows the state at point C of the optical signal from theterminal station B which is combined by the branching unit 1100 andamplified by an optical amplifier 1103. FIGS. 1C and 1D show the casewhere the power level of an optical signal is equal to that of eachother when the optical signal from the terminal station B is combinedwith the optical signal from the terminal station A. In this case, anoptical signal having any wavelength indicates the same S/N ratio to theASE noise as shown in FIG. 1D.

[0013]FIG. 1E also shows the state of the optical signal at point B. Inthis case, the power level of the optical signal from the terminalstation B is high. When the power level of the optical signal from theterminal station B is high, the state of the optical signal at point Cafter being combined by the branching unit 1100 and being amplified bythe optical amplifier 1103, becomes as shown in FIG. 1F. Therefore,although the S/N ratio of wavelength λ₁ is high, because the operationcharacteristics of the optical communications system are based on thelower S/N ratio, when the S/N ratios of the other wavelengths are low,the system is recognized as poor in operation characteristics.

[0014]FIGS. 2A, 2B, 3A, and 3B show the operation of the opticalamplifier and the S/N ratio.

[0015] In this example, the two optical signals having differentwavelengths are multiplexed, and an optical signal of a total of 0 dBmpower is input to the optical amplifier. The optical amplifier includesan automatic output level control circuit having a gain of 10 dBm and anoptical output is limited to 10 dBm. The state of the optical signal atthe input terminal is −3 dBm each for the power of the optical signalsof two wavelengths, a total of 0 dBm as shown in FIG. 2A. FIG. 2B showsthe output when such optical signals are input to the optical amplifier.That is, the optical signal of each wavelength is amplified, and thepower of each optical signal is +7 dBm with a total power of the outputlight indicating +01 dBm. On the other hand, the ASE noise is alsoamplified, and the S/N ratio to the ASE noise of each optical signal is30 dB. Therefore, the operation characteristic of the optical amplifierindicates the S/N ratio of 30 dB.

[0016]FIGS. 3A and 3B show the case where an input optical signal ismultiplexed with an optical signal having a different power level. Thecharacteristic of the optical amplifier is the same as that of theoptical amplifier shown in FIGS. 2A and 2B. However, as shown in FIG.3A, a total power of the optical signals having two differentwavelengths is 0 dBm with the power level of one optical signalindicating −1.5 dBm while the other optical signal indicating −4.5 dBm.There is 3 dB difference between the power levels. If such opticalsignals are input, the output is obtained as shown in FIG. 3B. That is,the higher power level of the optical signal between the two inputsignals is +8.5 dBm while the lower power level of the optical signal is+5.5 dBm because the optical signal having each wavelength is amplifiedsuch that the total power level of the output signals can be the abovedescribed value, that is, the output of the optical amplifier is fixedto +10 dBm. At this time, the ASE noise is amplified and the S/N ratiosare different between the wavelengths. That is, the S/N ratio of thewavelength indicating the higher power level is an acceptable valuewhile the S/N ratio of the wavelength indicating the lower power levelis relatively undesired. Since the operation characteristic of theoptical amplifier is evaluated by the undesired S/N ratio, theperformance of the optical amplifier is considered to be poor.

[0017] In the optical add-drop system as described above by referring toFIG. 1A, a lot of optical amplifiers are inserted between the terminalstation and the branching unit. In the branching unit, an independentlygenerated optical signal from the terminal station A is combined with anoptical signal from the terminal station B, and amplified by the opticalamplifier. The optical signals of respective wavelengths from theterminal stations A and B may not match in power when they are combinedbecause of the transmission distance and the difference in output.Furthermore, the power level of the optical signal may not be controlledjust as designed even if the system has been formed by carefullycomputing the output power and the attenuation of the optical signal inthe designing step. In this case, there arises a difference in S/N ratiobetween the optical signal having a lower power level and the opticalsignal having a higher power level after the amplification through theoptical amplifier as described by referring to FIGS. 2A, 2B, 3A, and 3B.The operation characteristic of the system is evaluated by the S/N ratioof the optical signal having the lower power level, that is, theundesired S/N ratio.

[0018] When the power level of the optical signal from a branch stationis different from that of the optical signal from the transmittingstation, the evaluation is made based on the lower S/N ratio indicatingthe transmission characteristic of the optical signal, therebyconsidering the system to be poor in performance.

SUMMARY OF THE INVENTION

[0019] The present invention aims at providing an optical communicationssystem capable of compensating the difference between the power level ofthe optical signal from the transmitting station and the optical signalfrom the branch station, and maintaining a high system performance.

[0020] The optical communications system according to the presentinvention includes a transmitting station for transmitting awavelength-multiplexed optical signal; a receiving station for receivingthe optical signal; a branch station for receiving an optical signalhaving a specific wavelength in the wavelength-multiplexed opticalsignals and transmitting the optical signal on the specific wavelength;and a branching unit for branching the optical signal having thespecific wavelength from the optical signal transmitted from thetransmitting station, transmitting it to the branch station, andcombining the optical signal transmitted from the branch station withthe optical signal which has the wavelength other than the specificwavelength and has been transmitted from the branch station. The signalsare combined with their power levels matching each other.

[0021] In an optical communications system including a transmittingstation for transmitting a wavelength-multiplexed optical signal; areceiving station for receiving the optical signal; a branch station forreceiving an optical signal having a specific wavelength in thewavelength-multiplexed optical signals and transmitting the opticalsignal on the specific wavelength; and a branching unit for branchingthe optical signal having the specific wavelength from the opticalsignal transmitted from the transmitting station, transmitting it to thebranch station, combining the optical signal transmitted from the branchstation with the optical signal from the transmitting station; andtransmitting the result to the receiving station, the branching unitaccording to the present invention branches the optical signal havingthe specific wavelength from the optical signal transmitted from thetransmitting station, transmits it to the branch station, and combinesthe optical signal transmitted from the branch station with the opticalsignal which has the wavelength other than the specific wavelength andhas been transmitted from the branch station. The signals are combinedwith their power levels matching each other.

[0022] Otherwise, the terminal station according to another aspect ofthe present invention includes an optical transmission signaltransmitting unit for generating an optical transmission signalmodulated using the data to be transmitted; a dummy light generationunit for generating a dummy light different in wavelength from theoptical transmission signal; a wavelength multiplexing unit forwavelength-multiplexing the dummy light and the optical transmissionsignal; and a level adjustment unit for adjusting the output level ofthe dummy light.

[0023] In the method of controlling the optical communications systemaccording to another aspect of the present invention with a systemincluding a first optical terminal station; a second terminal station, athird terminal station; an optical branching unit for connecting thefirst through third optical terminal station; and an optical amplifierfor maintaining an output signal at a constant level between the opticalbranching unit and the second optical terminal station wherein thebranching unit wavelength-multiplexes the optical transmission signalsfrom the first and second terminal stations and transmits the result tothe third terminal station, the second optical terminal station controlsthe optical transmission signal level of an output light from theoptical amplifier by transmitting the dummy light different inwavelength from the optical transmission signal and adjusting the levelof the dummy light.

[0024] Otherwise, in the terminal station in the optical communicationssystem according to the present invention with a system including afirst optical terminal station; a second terminal station, a thirdterminal station; an optical branching unit for connecting the firstthrough third optical terminal station; and an optical amplifier formaintaining an output signal at a constant level between the opticalbranching unit and the second optical terminal station wherein thebranching unit wavelength-multiplexes the optical transmission signalsfrom the first and second terminal stations and transmits the result tothe third terminal station, the second optical terminal station includesan optical transmission signal transmitting unit for generating anoptical transmission signal modulated using data to be transmitted; adummy light generation unit for generating a dummy light having awavelength different in wavelength from the optical transmission signal;a wavelength multiplexing unit for wavelength-multiplexing the dummylight and the optical transmission signal; and a level adjustment unitfor adjusting the output level of the dummy light.

[0025] In the optical communications system, the terminal station, orthe branching unit according to the present invention, when the opticalsignals in those transmitted from the transmitting station fortransmitting wavelength-multiplexed optical signals, but excluding thosehaving a specific wavelength to be transmitted to the branch station arecombined by the branching unit with the optical signals having thespecific wavelength transmitted from the branch station, the combinationcan be performed with the power levels of both optical signals matchingeach other. Thus, the difference in power level between the opticalsignals after the combination prevents the S/N ratio of the signal atthe lower power level from being lowered and the system performance frombeing deteriorated. That is, the present invention can realize anoptical add-drop system capable of applying the system performance at ahigh level f or a long time.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIGS. 1A through 1F are diagrams for explaining the conventionaladd-drop system and its problems;

[0027]FIGS. 2A and 2B are diagrams (1) for explaining the operation ofthe optical amplifier and the S/N ratio;

[0028]FIGS. 3A and 3B are diagrams (2) for explaining the operation ofthe optical amplifier and the S/N ratio;

[0029]FIG. 4 shows the first embodiment of the present invention;

[0030]FIG. 5 shows an example of the optical attenuator according to thefirst embodiment;

[0031]FIG. 6 shows the configuration according to the second embodimentof the present invention;

[0032]FIG. 7 shows the entire configuration according to the thirdembodiment of the present invention;

[0033]FIG. 8 shows the configuration for the add-drop of the opticalsignal in the branching unit according to the third embodiment of thepresent invention;

[0034]FIGS. 9A through 9C show the characteristic of the branching unitshown in FIG. 8;

[0035]FIGS. 10A and 10B show the state of the add optical signal inputto the branching unit in which the add optical signal is combined andthe output light from the branching unit to the receiving station;

[0036]FIGS. 11A and 11B show the case where a dummy light is used as acontrol means according to the third embodiment of the presentinvention;

[0037]FIG. 12 is a block diagram showing a part of the configuration ofthe terminal station as a transmitting station and a receiving station;and

[0038]FIG. 13 is a block diagram showing the configuration of a branchstation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039]FIG. 4 shows the first embodiment according to the presentinvention.

[0040]FIG. 4 shows the configuration in which a branching unit 16adjusts the power level of an optical signal from a branch station.Although only a down-line from the transmitting station to the receivingstation is shown in FIG. 4, an up-line from the receiving station to thetransmitting station is actually mounted.

[0041] The optical signal is transmitted from the transmitting station,wavelength-multiplexed, input to a circulator 10 and then to a fibergrating 11. In the fiber grating 11, only the optical signal having thewavelength to be transmitted to the branch station is reflected, andother signals pass straight. The optical signal reflected by the fibergrating 11 is input to the circulator 10 again and transmitted to thebranch station. When the optical signal passing straight through thefiber grating 11 also passes through an isolator 12 and a fiber grating13, enters a circulator 14, is combined with the optical signaltransmitted from the branch station, and is then transmitted to thereceiving station. When the optical signal transmitted from the branchstation is input to the circulator 14, it is transmitted to the fibergrating 13. Since the wavelength of the optical signal transmitted fromthe branch station is equal to that reflected by the fiber grating 11,it is also reflected by the fiber grating 13, input to the circulator 14again, and transmitted to the receiving side. The excess light which hasnot been reflected by the fiber grating 13 is prevented by the isolator12 from being propagated to the transmitting station. When the opticalsignal transmitted from the transmitting station is combined with theoptical signal transmitted from the branch station in the circulator 14,the S/N ratio is lowered when the signals are amplified by the opticalamplifier if there is a difference between the power levels of theoptical signals having respective wavelengths. In the case according tothe present embodiment, an optical attenuator 15 is provided in thetransmission line through which an optical signal is transmitted fromthe branch station. The optical attenuator 15 adjusts the power level ofthe optical signal from the branch station to make the optical signaltransmitted from the transmitting station match in power level theoptical signal transmitted from the branch station. Therefore, thesystem performance in the optical add-drop system can be maintainedhigh.

[0042]FIG. 5 shows an example of the optical attenuator according to thefirst embodiment of the present invention.

[0043] The optical attenuator shown in FIG. 5 is obtained by fusing thesingle mode fiber or the dispersion shifted fibers (DSF) 20 and 21 withtheir optical axes shifted from each other. The single mode fiber or theDSF 20 or 21 comprises cores 22 and 24, and cladding 23 and 25 forprotecting the cores 22 and 24. They are fused at a fusion portion 26.The core 22 and the core 24 are a little shifted from each other, andthere is a loss of light when an optical signal passes this portion.Therefore, the optical signal-after passing through this portion islower in power level than the optical signal detected before the opticalsignal passes through the portion. Therefore, the power level of theoptical signal transmitted from the branch station can be adjusted. Suchconnections of the optical fibers are referred to as an axis-shiftedsplice.

[0044] When the axis-shifted splice is used as a method of configuringthe optical attenuator, the connection between the optical fibers isfixed and the attenuation of an optical signal is also fixed. Therefore,the attenuation of the optical signal is adjusted by the axis-shiftedsplice only once when the system is designed. However, since the opticalattenuation can be maintained at a constant level for a long time, areliable optical attenuator can be obtained in the case where thebranching unit is provided underseas for use in underseas cablecommunications, and where the branching unit cannot be frequentlymaintained.

[0045] Because a unit which performs a splicing process usually containsequipment for detecting attenuation of the optical signal, the opticalattenuation is normally adjusted by adjusting the amount of the shift ofthe optical axis of the optical fiber while confirming the opticalattenuation when the optical fiber is spliced. Thus, an appropriateoptical attenuation can be realized.

[0046] The configuration of the optical attenuator is not limited to theabove described axis-shifted splice, but can be optionally determinedwithin the range normally anticipated by one of ordinary skill of theart.

[0047]FIG. 6 shows the configuration of the second embodiment of thepresent invention.

[0048] Also in the present embodiment, a branching unit 30 is designedin a way that the power level of the optical signal transmitted from thebranch station is adjusted to match the power level of the opticalsignal transmitted from the transmitting station. In FIG. 3, only thedown-line from the transmitting station is indicated. (Actually, therecan be an upline.)

[0049] The optical signal transmitted from the transmitting station isamplified by an optical amplifier 31, and branched by a coupler 32.Since the branching process in this example is performed to monitor thepower level of the optical signal from the transmitting station, thepower of the most optical signals is designed not to branched, but topass straight. From the optical signals which pass straight, the opticalsignals having the wavelengths to be transmitted in a circulator 33 anda fiber grating 34 to the branch station are retrieved, and theretrieved optical signals are transmitted to the branch station. Theoptical signals having the other wavelengths pass further straightthrough the isolator 35, and a fiber grating 36 and a circulator 37combine the optical signal from the branch station and transmit theresult to the receiving station.

[0050] The optical signal branched in coupler 32 is converted into anelectric signal by a photodiode 38 in a control circuit 400, and inputto a comparator 39. The optical signal transmitted from the branchstation is input to an optical amplifier 43, amplified, and branched bya coupler 44. At this point, most optical signals pass straight, and arecombined with the optical signals passing straight from the transmittingstation by the circulator 37 and fiber grating 36, and the result istransmitted to the receiving side. The optical signal branched in thecoupler 44 is converted into an electric signal by a photodiode 41 inthe control circuit 400. The power level of the signal converted into anelectric signal and received by a level converter 40 is adjusted andinput to the comparator 39. The level converter 40 is provided for thefollowing reason. That is, the optical signal received by the photodiode38 is transmitted after the optical signals having, for example, eightdifferent wavelengths are transmitted from the transmitting station andmultiplexed. However, the optical signals received by the photodiode 41are transmitted from the branch station, and contain the optical signalshaving, for example, four wavelengths in the eight different wavelengthsused in transmitting the optical signals from the transmitting station.Therefore, the optical signals received by the photodiode 38 contain 8optical signals while the optical signals received by the photodiode 41contain only four optical signals. If the power levels of these opticalsignals are directly compared, those received by the photodiode 38 arenaturally higher. However, it is necessary to make the power level ofeach wavelength of the optical signal transmitted from the branchstation match the power level of each wavelength of the optical signalwhich has been transmitted from the transmitting station and has notbeen dropped (retrieved) to the branch station. Therefore, the powerlevel of the 8-wave-multiplexed optical signal from the branch stationis converted by the level converter 40 to match the power level of the8-wave-multiplexed optical signal from the transmitting station. Then,the result is input to the comparator 39.

[0051] The comparator 39 compares the power levels of the thus obtainedelectric signals and the comparison result is input to an operationalamplifier 42. The comparison result is compared with the reference value(ref), and a control signal is issued to the optical amplifier 43 ifthere is a difference between the power level of the optical signal fromthe branch station and the power level of the optical signal from thetransmitting station so that the power level of the optical signal fromthe branch station can be adjusted in a way that the power level of eachwavelength of the optical signal passing straight from the transmittingstation can be made to match the power level of each wavelength of theoptical signal output from the optical amplifier 43.

[0052] Thus, when the optical signals are combined by the circulator 37and the fiber grating 36, the power levels of both optical signals canbe equal to each other. Therefore, the above described deterioration ofthe system performance caused by the optical amplifier while the opticalsignal is being transmitted to the receiving terminal can besuccessfully prevented.

[0053] The above described configuration of the branching unit is onlyan example, and there can be a number of variations which are implied bythe technological concept of the present embodiment.

[0054]FIG. 7 shows the third embodiment of the present invention.

[0055] According to the first embodiment and the second embodiment, thedifference in level between an optical transmission signal passingthrough the branching unit and an optical signal inserted from thebranch station is adjusted in the branching unit. On the other hand,according to the third embodiment, the level of an optical signal isadjusted under the control on the terminal side before the signal isinput to the branching unit.

[0056] Practically, a dummy light different in wavelength from anoptical transmission signal is transmitted, and the level of thetransmission signal is adjusted by changing the level of the dummy lightin an optical terminal station.

[0057] That is, by raising the level of the dummy light, the level ofthe optical transmission signal is lowered when it passes through theoptical amplifier. By lowering the level of the dummy light, the levelof the optical transmission signal is raised when it passes through theoptical amplifier.

[0058]FIG. 7 shows the configuration of the system comprising, at abranch station 53, an optical transmission signal transmitting unit 1-1;a dummy light generation unit 1-2 for generating a dummy light at anoptical terminal station; a control unit 1-6 for adjusting the level ofthe dummy light; and a wavelength multiplexing unit 1-4 for combiningthe optical signals of different wavelengths. With this configuration,the level of the optical transmission signal is adjusted.

[0059]FIG. 8 shows the third embodiment of the present invention.

[0060] The configuration of the system is as shown in FIG. 7, comprisingbranch station 53; an optical transmission signal transmitting unit 1-1;a dummy light generation unit 1-2 for generating a dummy light at acontrol unit 1-6 for adjusting the level of the dummy light; and awavelength multiplexing unit 1-4 for combining the optical signals ofdifferent wavelengths. With this configuration, the level of the opticaltransmission signal is adjusted.

[0061] A signal from the branch station 53 is amplified by the opticalamplifier 61-1 when it is transmitted to the optical terminal station Bthrough the optical amplifier 61-1. Thus, the optical signal level canbe changed based on the level of a dummy light.

[0062] By raising the level of the dummy light, the level of the opticaltransmission signal is lowered when it passes through the opticalamplifier 61-1. By lowering the level of the dummy light, the level ofthe optical transmission signal is raised when it passes through theoptical amplifier 61-1.

[0063] That is, when the optical signals having wavelengths differentfrom each other in power level are input to the optical amplifier, theoutput of the optical amplifier can be set constant as described in thedescription of the prior art. As a result, there arises a difference inpower level after the amplification between the optical signals one ofwhich indicates a high power level while the other indicates a low powerlevel when input to the optical amplifier. Based on this, a dummy lightcapable of being variable in output level and different in wavelengthfrom an optical signal, is transmitted together with the optical signalcontaining information data when the optical signal is transmitted fromthe branch station. Thus, the power level of the optical signal can beadjusted when the optical signal passes through optical amplifiers 60-1through 60-n and 61-1 through 61-n.

[0064]FIG. 7 shows the entire configuration according to the thirdembodiment of the present invention.

[0065]FIG. 7 shows the configuration of the optical add-drop system inwhich terminal stations A and B are connected to each other using anup-line and a down-line through a branching unit 51. Also, a line isbranched from the branching unit 51, and an up-line and a down-line areprovided so that a branch station 53 can transmit and receive an opticalsignal. The transmission lines for connection of the terminal stationsA, B, the branch station 53, and the branching unit 51 are provided withoptical amplifiers 55-1 through 55-n, 56-1 through 56-n, 57-1 through57-n, 58-1 through 58-n, 59-1 through 59-n, 60-1 through 60-n, 61-1through 61-n, and 62-1 through 62-n, each of which has an ALC circuit,thereby amplifying the optical signal when the optical signal istransmitted over a long distance. The branching unit 51 has an up-lineand a down-line. The up-line comprises an optical circulator 33 forinputting an optical transmission signal from the optical terminalstation A to a fiber grating 34 and transmitting an optical transmissionsignal having a specific wavelength from the fiber grating 34 to abranch station 53; an optical isolator 35 for passing a light which haspassed through the fiber grating 34; and an optical circulator 37 forinputting the optical signal from the branch station 53 to the fibergrating 36 and outputting a light reflected from the optical isolator 35and the fiber grating 36 to the optical terminal station B side. Thedown-line comprises an optical circulator 33′ for inputting an opticaltransmission signal from the optical terminal station B to a fibergrating 34′ and transmitting an optical transmission signal having aspecific wavelength from the fiber grating 34′ to a branch station 53′;an optical isolator 35′ for passing a light which has passed through thefiber grating 34′; and an optical circulator 37′ for inputting theoptical signal from the branch station 53′ to the fiber grating 36′ andoutputting a light reflected from the optical isolator 35′ and the fibergrating 36′ to the optical terminal station B side.

[0066] The branch station 53 comprises a control unit 1-6 for receivingan optical transmission signal from the down-line; the opticaltransmission signal transmitting unit 1-1 for transmitting an opticaltransmission signal; the dummy light generation unit 1-2 for changingthe level of a dummy light upon receipt of the signal from the receivingunit 1-6; the wavelength multiplexing unit 1-4 forwavelength-multiplexing the output from the dummy light generation unit1-2 and the optical transmission signal transmitting unit 1-1; a controlunit 1-6′ for receiving an optical transmission signal from the up-line;the optical transmission signal transmitting unit 1-1′ for transmittingan optical transmission signal; the dummy light generation unit 1-2′ forchanging the level of a dummy light upon receipt of the signal from thecontrol unit 1-6′; and the wavelength multiplexing unit 1-4′ forwavelength-multiplexing the outputs from the dummy light generation unit1-2′ and the optical transmission signal transmitting unit 1-1′.

[0067] The adjustment between the power level of the optical signal fromthe branch station 53 and the power level of the optical signal from theterminal station A or B is made using dummy light generation units 1-2and 1-2′ provided in the branch station 53, optical spectrum analyzers65 and 66 provided in the terminal stations A and B. The dummy lightgenerated by the dummy light generation units 1-2 and 1-2′ should bedifferent in wavelength from the optical signal.

[0068] That is, when a dummy light is multiplexed and transmitted withthe optical signal transmitted from the branch station 53, the output ofthe optical signal can be adjusted depending on the power level of thedummy light when they pass through the optical amplifier. For example,when the power level of the optical signal is higher than that of thedummy light, the output of the optical signal is larger than that of thedummy light after it is amplified by the optical amplifier. On the otherhand, when the power level of the dummy light is higher than that of theoptical signal, the output of the dummy light is larger than that of theoptical signal after it is amplified by the optical amplifier, and theoptical signal indicates a lower power level. Since the output of theoptical signal remains constant, the sum of the output power of thedummy light and the output power of the optical signal should beconstant. Therefore, changing the power level of the dummy light alsochanges the power level of the optical signal output from the opticalamplifier.

[0069] The terminal stations A and B for receiving an optical signal areprovided with the optical spectrum analyzers 65 and 66 for detecting thepower levels of the signals having respective wavelengths in thereceived optical signal. It is determined whether or not there is adifference in power level by detecting the power level of eachwavelength of the optical signal transmitted from the branch station 53and the optical signal directly transmitted from the terminal station Aor B. The result is transmitted to the branch station 53 with an opticalsignal. If the control units 1-6 and 1-6′ of the branch station 53recognize that the optical signal transmitted by the branch station 53is different in power level from the optical signal directly transmittedfrom the terminal station A or B, then the power level of the dummylight generation units 1-2 and 1-2′ is adjusted so that the power levelof the optical signal transmitted from the branch station 53 and outputfrom the optical amplifier can be adjusted. Thus, the power level of theoptical signal having each wavelength is constantly monitored by thereceiving terminal station, and the power level of the dummy light isadjusted by the branch station 53 so that power level of the opticalsignal transmitted from the branch station 53 and the power level of theoptical signal directly transmitted from the terminal station A or B canbe approximately equal to each other when they are combined by thebranching unit 51. Therefore, a high system performance can bemaintained without deteriorating the operation characteristics as asystem only because the optical signal having optically-multiplexedwavelengths indicates a low power level and then a deteriorated S/Nratio.

[0070]FIG. 8 shows the configuration for the add-drop of the opticalsignal in the branching unit according to the third embodiment of thepresent invention.

[0071] In FIG. 8, the units in the up-line are omitted. The branchingunit according to the third embodiment has only the function ofperforming the add-drop of the optical signal. That is, the opticalsignal transmitted from the transmitting station andwavelength-multiplexed passes through a circulator 70 and input to fibergratings 73-1 through 73-4. Each of the fiber gratings 73-1 through 73-4functions to reflect the optical signal having unique wavelength. Thatis, the fiber gratings 73-1, 73-2, 73-3, and 73-4 selectively reflectthe optical signals having the wavelength λ₁, λ₂, λ₃, and λ₄respectively from the optical signal transmitted from the transmittingstation, and input the respective optical signals to the circulator 70again. The optical signal reflected by the fiber gratings 73-1 through73-4 enters the circulator 70 again, takes a different path, and istransmitted to the branch station as a drop optical signal. The opticalsignal not reflected by the fiber gratings 73-1 through 73-4 passesthrough an isolator 72, and fiber gratings 74-1 through 74-4, enters acirculator 71, is combined with the add optical signal transmitted fromthe branch station, and is transmitted to the receiving station.

[0072] The add optical signal and a dummy light transmitted from thebranch station are input to the circulator 71, and are transmitted tothe fiber gratings 74-1 through 74-4. As described above, the opticalsignals having wavelengths λ₁ through λ₄ are reflected, input to thecirculator 71 again, and transmitted to the receiving station. At thistime, the dummy light transmitted together with the optical signal as anadd signal is not reflected by the fiber gratings 74-1 through 74-4 norpasses through the isolator 72. Thus, most of the signals are dispersed.With this configuration, the dummy light is not transmitted to thereceiving station side.

[0073]FIGS. 9A through 9C show the characteristics of the branching unitshown in FIG. 8.

[0074]FIG. 9A shows the passage characteristics from the transmittingstation to the receiving station. The incident light from thetransmitting station is white light, and FIG. 9A indicates thetransmission characteristic around the isolator 72 (FIG. 8). FIG. 9Aindicates that the optical transmission intensity is lowered around fourcentral wavelengths. It implies that the fiber gratings 73-1 through73-4 reflect the light having these wavelengths, and the light is notoutput to the isolator 72. The wavelength other than a specificwavelength keeps unchanged in intensity. Therefore, with theconfiguration shown in FIG. 8, only the optical signal having a specificwavelength can be selectively prevented from passing.

[0075]FIG. 9B shows the characteristic of the drop of the optical signalfrom the transmitting station to the branch station. The light from thetransmitting station is white light. FIG. 9B indicates that the lighthaving the wavelength of low transmittance shown in FIG. 9A is retrievedon the contrary, and is transmitted to the branch station. The lighthaving four different wavelengths is reflected by fiber gratings 73-1through 73-4 shown in FIG. 8, and is transmitted to the branch stationby the circulator 70.

[0076]FIG. 9C shows the transmission characteristics of the opticalsignal from the branch station to the receiving station. In this case,no lights are input from the transmitting station, and a white light isinput from the branch station to check what type of wavelength isdetected. In this case, the light input from the branch station istransmitted by the circulator 71 to the fiber gratings 74-1 through74-4, and the light having the same wavelength as the case shown in FIG.9B is reflected. Then, the light is input to the circulator 71 again,and output to the receiving station. As shown in FIG. 9C, the lighthaving four different wavelengths is output, and the other lights areoutput only as low-level noises.

[0077]FIGS. 10A and 10B show the add optical signal input to thebranching unit, and show the state of the output light from thebranching unit, in which the add optical signal is combined, to thereceiving station.

[0078] As shows FIG. 10A, it is assumed that a 4-wave multiplexed signalis transmitted from the branch station. If the signal is combined withthe optical signal from the transmitting station without control as inthe conventional technology, the combination results as shown in FIG.10B. In this example, an 8-wave multiplexed signal is transmitted fromthe transmitting station, and it is assumed that the optical signal (1)having 4 shorter wavelengths shown by FIG. 10A is add-dropped by thebranching unit.

[0079] If the optical signal from the branch station as shown in FIG.10A is combined with the optical signal (2) from the transmittingstation without any control, there arises a difference in power levelbetween the optical signal (2) passing from the transmitting station andthe optical signal (1) from the branch station as shown in FIG. 10Bbecause the power level of the optical signal from the branch station isdifferent from the power level of the optical signal from thetransmitting station. In the case shown in FIG. 10B, the power level ofthe optical signal from the branch station is higher. That is, theoptical signal shown by (1) is an optical signal from the branch stationwhereas the optical signal shown by (2) is an optical signal passing inthe branching unit toward the receiving station. In FIG. 10A and 10B,the mound-shaped portion indicates the ASE noise.

[0080]FIGS. 11A and 11B show the case where a dummy light is used as acontrol means according to the third embodiment of the presentinvention.

[0081]FIG. 11A indicates an optical signal containing a dummy lighttransmitted from the branch station to the branching unit. (4) indicatesan optical signal containing information. (3) indicates a dummy light.As clearly recognized by comparing FIG. 11A with FIG. 10A, if the powerlevel of the dummy light (3) is higher, then the power level of theoptical signal (4) containing information becomes relatively low. In thecase shown in FIG. 10B, the optical signal at a higher power level fromthe branch station is combined. Therefore, a difference in power leveloccurs between the optical signal from the branch station and theoptical signal from the transmitting station directly to the receivingstation. However, the power level of the optical signal (4) containingthe information from the branch station can be lowered using the dummylight (3) as shown in FIG. 11A. Therefore, the difference in level canbe reduced almost down to zero between the optical signal (5) outputfrom the transmitting station to the receiving side in the branchingunit and the optical signal (4) transmitted from the branch station asshown in FIG. 11B. The optical signal corresponding to (3) shown in FIG.11B is the dummy light which has not been completely dispersed in thebranching unit and has been output to the receiving station.

[0082] In the above described example, the power level of the opticalsignal containing the information transmitted from the branch station isrelatively higher. If its power level is lower, the optical signal (4)containing the information can be relatively higher by lowering thepower level of the dummy light. Therefore, the power level of theoptical signal (5) from the transmitting station can be made to matchthe power level of the optical signal (4) from the branch station byadjusting the output of the dummy light in the branch station dependingon the situation, thereby maintaining the high system performance.

[0083] The above described technology can be applied to the opticalwavelength multiplexing communications indicating a higher multiplicityother than the 8-wave transmission, or to the optical wavelengthmultiplexing communications indicating a lower multiplicity. Thewavelength of the dummy light does not have to be necessarily shorter,but can be any type as long as it is in the range of the band used toamplify an optical signal in the optical amplifier.

[0084]FIG. 12 is a block diagram showing a part of the terminal stationas a transmitting station and a receiving station.

[0085] If the optical signal is transmitted from the branching unitthrough the up-line, then a coupler 90 partly branches the opticalsignal. The coupler 90 branches the optical signal at the rate of, forexample, 10:1. Most optical signals pass through the coupler 90 and isbranched by couplers 91 and 92. Each of the branched optical signals isextracted as the signal having each wavelength (a optical signal alongeach channel) through optical filters 93-1 through 93-3. The opticalsignal having each wavelength is amplified by preamplifiers 94-1 through94-3, received by optical receivers 95-1 through 95-3, and convertedinto an electric signal. A demultiplexer 97 retrieves the informationand transmits it to the information processing unit not shown in FIG.12.

[0086] The optical signal branched by the coupler 90 is input to anoptical spectrum analyzer 96, and the power level of the optical signalof each wavelength is checked. The reception level difference of theoptical signal is retrieved as information. It is written to theinformation communications format of the optical signal (for example,POH (pass overhead) of SDH/SONET) in the data format generation unit notshown in FIG. 12, and an electric signal is generated in a way that itis applied to the format with other information signals. These processesare performed by a multiplexer 98 shown in FIG. 12. The data signaloutput from the multiplexer 98 is converted into the optical signalshaving respective wavelengths by optical transmitting units 99-1 through99-3 provided for each channel, each optical signal having eachwavelength amplified using post amplifiers 100-1 through 100-3, andtransmitted. Thus generated optical signal having each wavelength iscombined by couplers 101 and 102, and transmitted to a terminal stationor a branch station through the down-line.

[0087] The terminal station which receives the optical signal not onlyretrieves the information contained in the optical signal, but alsodetects the difference in power level of the optical signal having eachwavelength using the optical spectrum analyzer 96, and transmits itagain as the information inserted into a part of the main signal.

[0088] In FIG. 12, the number of times the wavelength of an opticalsignal is multiplexed is 3, but the number is not limited to 3. Based onFIG. 12, of the three optical receivers 95-1 through 95-3, two opticalreceivers receive the optical signal from the branch station, and theremaining one optical receiver receives the optical signal which istransmitted from the terminal station, that is, a transmitting station,and passes through the branching unit. Similarly, of the opticaltransmitting units 99-1 through 99-3, two units transmit the opticalsignal having a wavelength for transmission to the branch station, andthe remaining one unit transmits the optical signal having a wavelengthfor transmission to the terminal station, that is, the receivingstation.

[0089]FIG. 13 is a block diagram showing a part of the branch station.

[0090] Through the down-line, the optical signals having two differentwavelengths (not limited to two) is transmitted from the branching unit.A coupler 118 branches the optical signal. Optical filters 119-1 and119-2 extract an optical signal having each wavelength. The opticalsignals having respective wavelengths are amplified by preamplifiers120-1 and 120-2, and converted into electric signals by opticalreceivers 121-1 and 121-2. Demultiplexer 122 retrieves the informationand transmits the information to the information processing unit notshown in FIG. 13.

[0091] The demultiplexer 122 extracts the information written in thedata transmission format of the optical signal obtained by the opticalspectrum analyzer (for example, the reception level difference of anoptical signal of each wavelength transmitted from the terminal stationin the POH area in the SDH/SONET) in the terminal station, and transmitsthe information to the computer 117.

[0092] The computer 117 sends the data information to be transmitted asa signal to the optical transmitters 114-1 and 114-2, and the opticalsignal having each wavelength is generated. Furthermore, the branchstation comprises a dummy light generation unit 115 to output a dummylight. The optical signal having each wavelength and the dummy light areamplified by post amplifiers 113-1 through 113-3, and combined bycouplers 111 and 112 for transmission. The combined optical signal isbranched by a coupler 110. The coupler 110 branches a light at the rateof, for example, 10:1, passing most of the light as is, and branching asmall part of it. The optical signal branched by the coupler 110 isinput to an optical spectrum analyzer 116, and the power leveldifference of each wavelength of the optical signal output from thebranch station is detected.

[0093] The detection result of the optical spectrum analyzer 116 isinput to the computer 117, and is compared with the information aboutthe reception level difference at the terminal station extracted by thedemultiplexer 122. The control signal of the transmission power of thedummy light is transmitted to the post amplifier 113-3. Thus, thereception level difference between the optical signal transmitted fromthe branch station and the optical signal transmitted to the receivingstation without dropping between the terminal station and the branchingunit can be monitored. Based on the monitor result, the transmissionpower level of the dummy light is adjusted. As a result, the differencein power level between the optical signal transmitted from the branchstation and combined by the branching unit and the optical signal notdropped can be controlled to be reduced down to almost zero. Therefore,the deterioration in S/N ratio from the power-level-difference can beprevented, and the high system performance can be maintained.

[0094] As shown in FIG. 13, the optical signal transmitted to the branchstation is carried with two different wavelengths. However, the systemconfiguration is not limited to this application, but the wavelengthmultiplicity of the optical signal transmitted from the terminal stationand the wavelength multiplicity of the optical signal transmitted to thebranch station should be appropriately determined as necessary in eachdesigning step.

[0095] According to the present invention, the difference in power levelbetween the optical signal having each wavelength transmitted from thebranch station in a branching unit and the optical signal of eachwavelength not dropped can be compensated when they are combined. Thesystem performance can be prevented from being lowered by thedeterioration of the S/N ratio of the lower power level. Therefore, theoptical add-drop system capable of maintaining a high system performancecan be provided.

In the claims:
 1. An optical communications system, comprising: atransmitting station transmitting a wavelength division multiplexed(WDM) optical signal which includes first and second optical signals atdifferent wavelengths multiplexed together; a branch stationtransmitting an optical signal with the same wavelength as the secondoptical signal in the WDM optical signal; a branching unit branching thesecond optical signal from the WDM optical signal transmitted by thetransmitting station, transmitting the branched second optical signal tothe branch station, and combining the optical signal transmitted by thebranch station with the WDM optical signal having the second opticalsignal branched therefrom, to produce a combined signal; a receivingstation receiving the combined signal; and a power level controllercausing power levels in the combined signal of the first optical signaland the optical signal transmitted by the branch station to be equal. 2.The optical communications system according to claim 1, wherein thepower level controller comprises an optical attenuator adjusting thepower level of the optical signal transmitted by the branch station. 3.The optical communications system according to claim 1, wherein thepower level controller compares the power level of the WDM opticalsignal with the power level of the optical signal transmitted by thebranch station, and adjusts the power level of the optical signaltransmitted by the branch station based on a comparison result.
 4. Theoptical communications system according to claim 3, wherein the powerlevel controller comprises: an optical amplifier capable of adjusting again; and a gain adjustor adjusting the power level of the opticalsignal transmitted by the branch station by changing the gain of theoptical amplifier.